The proposed study is the second stage of a research program designed to examine the therapeutic potential of reducing dietary fat as a means of producing a sustained weight loss. A low-fat diet (25-30 percent of calories) and a medium-fat diet (35-40 percent of calories) will be administered to the fourteen overweight female subjects, using a balanced cross-over design. The two dietary regimens will be matched for palatability and will contain the same food items, differing almost exclusively in their fat content. During each of the two 10-week treatment periods, subjects will obtain all of their food from the Cornell Human Research Unit, where daily food intake will be measured. The experimental conditions are designed to promote spontaneous food intake throughout the day and to interfere minimally with the subjects' daily routines. This study is intended to answer the following questions: a) Will subjects respond to the decreased caloric density of the low-fat diet by increasing their food intake? b) If the subjects do increase their food intake, will the increase be sufficient to completely compensate for the reduced caloric concentration of the low-fat diet? and c) How much weight (body fat) will be lost merely by reducing dietary fat, under conditions that impose no restriction on the amount of food consumed? The results of this study should have both basic scientific and applied value. They will provide an assessment of the precision of caloric regulation that should be more representative of "free- living" humans than that provided by previous research. In addition, if reducing dietary fat produces a sustained loss in caloric intake and body weight (as indicated by our previous study utilizing 2-week treatments), this knowledge would provide an added incentive to the weight-conscious public to adhere to a diet that would not only facilitate weight reduction, but may also reduce the risk of cancer and heart disease.